1. Field of the Invention
This invention relates to semiconductor fabrication technologies, and more particularly, to a double-alternating phase-shifting mask (PSM) for use in photolithography in semiconductor fabrication processes. PSM can eliminate ghost lines that would otherwise occur due to side-lobe effect in the resulted pattern definition.
2. Description of Related Art
In semiconductor fabrication, photolithography is an important and indispensable technique which is used to transfer circuit layout patterns through a mask onto predefined locations on a semiconductor wafer. Many processes in semiconductor fabrications, such as etching and ion implantation, require the use of photolithography. In a photolithographic process, resolution and depth of focus (DOF) are two major checkpoints used to appraise the quality of the pattern definition. A high level of integration requires a high resolution of pattern definition since the feature size is very small. To increase the resolution, a laser source with a very short wavelength, such as a krypton (Kr) deep ultraiolet laser with a wavelength of 2,480 .ANG. (angstrom), is used as the exposure light in the photolithographic process. The use of a short-wavelength exposure light, however, will result in a shallow DOF. To allow high resolution and good DOF, one solution is to use the so-called phase-shifting mask (PSM).
Fundamentally, a PSM is formed by adding phase shifter layers on a conventional mask that can cause destructive interference to the light passing through it such that the contrast and resolution of the resulting pattern definition can be increased. One benefit of the PSM is that it can increase the resolution of pattern definition without having to change the wavelength of the exposure light.
In semiconductor fabrications, hole patterns are usually required to define contact holes in the wafer. Conventionally, the so-called half-tone phase shift mask (HTPSM) is used to define contact hole patterns with a large DOF. A conventional HTPSM photolithographic process is illustratively depicted in the following with reference to FIGS. 1A-1C, wherein FIG. 1A is a schematic top view of the conventional HTPSM, FIG. 1B is a cross-sectional view taken along the line I--I of the conventional HTPSM in FIG. 1A and FIG. 1C is a graph showing the distribution of light intensity over a wafer from an exposure light passing through the conventional HTPSM of FIG. 1B.
As shown, the conventional HTPSM includes a quartz substrate 14 and a shifter layer 12 formed over the quartz substrate 14. The shifter layer 12 can be, for example, a layer of MoSi.sub.z O.sub.x N.sub.y. Since positive photoresist is typically used in the photolithographic process for forming contact holes, the portions of the quartz substrate 14 that are uncovered by the shifter layer 12, as designated by the reference numeral 10, are defined as contact hole patterns used for pattern definition of contact holes. The shifter layer 12 has light transmittance of from 3% to 10% and is capable of causing a phase shift of 180.degree. to the light passing through it. By contrast, the light passing through the contact hole patterns 10 has no shift in angle. The phase shift and the intensity of the light passing through the conventional HTPSM are illustrated in FIG. 1C. As shown, the light passing through the contact hole patterns 10 is not shifted in phase and has a high intensity, while the light passing through the shifter layer 12 is shifted in phase by 180.degree. and is attenuated in intensity. Therefore, near the borderlines between the contact hole patterns 10 and the shifter layer 12, the light passing through the transparent contact hole patterns 10 and the light passing through the shifter layer 12 are subjected to destructive interference, thus forming a zero point that would increase the contrast of the resulted pattern definition on the photoresist layer. The resulted pattern definition is thus higher in resolution.
The foregoing conventional HTPSM, however, has the drawback of a side-lobe effect due to the diffractions of the exposure light passing through the contact hole patterns 10, which is particularly noticeable when the duty ratio (the ratio of the size of each contact hole pattern to the separation between two neighboring contact hole patterns) is close to one. The side-lobe effect can cause ghost lines between those locations on the wafer where the contact holes are defined. In the case of FIG. 1A, for example, the exposure light passing through the contact hole patterns 10a, 10b, 10c, 10d can cause the appearance of ghost lines at a point on the wafer corresponding to the point C between the contact hole patterns 10a, 10b, 10c, 10d.